GASTROPODA 541 



2. LACUNA Turton. Shell conical and thin with about 5 whorls, 

 with a long umbilical groove along the columella leading to a deep 

 umbilicus: 16 species. 



L vincta (Montagu) (Fig. 839). Shell gray or yellowish in color, 

 with 2 dark bands on the upper whorl and 4 on the lower one, which 

 may be wanting; length 12 mm.; diameter 7 mm.: circumpolar, south 

 to Long Island Sound and Puget Sound; very common; Europe. 



FAMILY 4. KISSOIDAE. 



Shell small, spiral, turreted; aperture regularly rounded, with a 

 slightly-expanded lip; operculum horny; eyes at the base of the ten- 

 tacles; epipodium with filaments: marine or brackish water; over 200 

 species, principally towards the south. 



RISSOA Freminville. Shell minute, conical, multispiral, white in 

 color: 75 species, mostly in the north temperate zone in 

 deep water. 



R. minuta (Totten). Shell thin and polished, with 5 

 whorls and an obtuse apex; color yellowish, which is 

 usually coated with a green pigment ; length 4 mm. ; diam- 

 eter 2 mm. : New Jersey to Gulf of St. Lawrence ; common 

 on seaweed and under stones. 



R. aculeus (Gould) (Fig. 840). Shell thin, with 6 whorls, light yel- 

 lowish in color, with microscopic revolving lines ; length 4 mm. ; diameter 

 1.7 mm.: Long Island Sound to Greenland; common on seaweed. 



FAMILY 5. HYDEOBIIDAE. 



Shell small, usually elongate and conical, with a pointed spire; 

 aperture rounded; operculum corneous or calcareous; umbilicus usually 

 open: about 80 species, in fresh and brackish water; cosmopolitan. 



Key to the genera of Hydrobiidae here described: 



Oj Shell ovate. 



6 t Umbilicus closed. 



Cj In eastern and central states 1. BYTHINIA 



c 2 On Pacific slope 3. FLUMINICOLA 



6 a Umbilicus open. 



<?! Spire well developed 2. AMNICOLA 



c 2 Spire very low 4. SOMATOGYBUS 



a a Shell elongate. 



&! Umbilicus narrow and deep 5. POMATIOPSIS 



6 a Umbilicus absent or very shallow 6. PALUDESTBINA 



1. BYTHINIA Gray. Shell conical, thin, covered with a thin perios- 

 tracum; operculum calcareous, not spiral, but with prominent growth- 

 ridges: in fresh water; 50 species, all in the Old World. 



